Lambda(二)lambda表達式使用

Lambda(二)lambda表達式使用

Lambda 表達式組成：

/*
    param list   arrow        lambda body
    (o1,o2)       ->       o1.getColor().CompareTo(o2.getColor());
*/

Lambda表達式需要有與之相匹配的預定義函數式介面：

/*
FunctionalInterface介面：Predicate<T> 
方法：Boolean test(T t);

FunctionalInterface介面：Consume<T> 
方法：void accept(T t);

FunctionalInterface介面：Function<T,R> 
方法：R apply(T t);

FunctionalInterface介面：Suppiler<T> 
方法：T get();
*/

 簡單使用案例，source code 如下

Consumer<String> c = s -> System.out.println(s);

Function<String,Integer> lambda =  s->s.length();

Predicate<Apple> predicate = a->a.getColor().equals("green");

Function<Apple,Boolean> function = a->a.getColor().equals("red");

Supplier<Apple> instance = Apple::new;

假如，現在要對Apple的list進行排序（常規vsLambda）：

//implement item1
list.sort(new Comparator<Apple>() {
            @Override
            public int compare(Apple o1, Apple o2) {
                return o1.getColor().compareTo(o2.getColor());
            }
        });

//implement item2
list.sort((o1, o2) -> o1.getColor().compareTo(o2.getColor()));

//implement item2
list.sort(comparing(Apple::getColor));

自定義使用，source code如下

public static List<Apple> filter(List<Apple> apples, Predicate<Apple> predicate){
        List<Apple> result = new ArrayList<>();
        for(Apple a:apples){
            if(predicate.test(a)){
                result.add(a);
            }
        }
        return result;
    }

public static void main(String[] args) {
        List<Apple> apples = Arrays.asList(new Apple("green", 120), new Apple("red", 150));
        List<Apple> green = filter(apples, a -> a.getColor().equals("green"));
        System.out.println(green);
    }

　　java 8 API中最多只有兩個入參，假如有多個參數，可以自己定義，source code如下

//自定義FunctionalInterface介面
@FunctionalInterface
public interface ThreeFunction<T,U,K,R> {
    R apply(T t,U u,K k);
}
//具體使用
ThreeFunction<String,Long,String,ComplexApple> tf = ComplexApple::new;
ComplexApple cp = tf.apply("yellow", 111L, "fushi");
System.out.println(cp);

　　方法推導（MethodReference）：

/**
 * 使用方法推導的條件：
 *      1、類的實例方法
 *      2、對象的實例方法
 *      3、構造函數
 */
public static <T> void useConsume(Consumer<T> consumer,T t){
        consumer.accept(t);
}
useConsume(System.out::println,"chuangg");

List<Apple> apples = Arrays.asList(new Apple("red", 100),
                new Apple("green", 150),
                new Apple("abc", 110));
apples.stream().forEach(System.out::println);
//>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
int i = Integer.parseInt("123");

Function<String,Integer> f = Integer::parseInt;
Integer apply = f.apply("123");
System.out.println(apply);

BiFunction<String, Integer, Character> f1 = String::charAt;
Character r1 = f1.apply("hello", 2);
System.out.println(r1);

String s = new String("hello");
Function<Integer, Character> f2 = s::charAt;
Character r2 = f2.apply(1);
System.out.println(r2);

Supplier<Apple> appleSupplier = Apple::new;
Apple apple = appleSupplier.get();

BiFunction<String,Long,Apple> appleBiFunction = Apple::new;
Apple blue = appleBiFunction.apply("blue", 123L);
System.out.println(blue);